The proliferation of electronic devices over the past several years has created problems of radiation containment from these devices. When a digital device changes voltage, it emits electrical and magnetic energy which is comparable to frictional heat loss in any mechanical machine. When not controlled, this radiation can interfere with the normal operating use of other electrical devices. In addition, many devices need to be shielded against incoming radiation to prevent damage to the product itself The prevention of egress or ingress of electromagnetic radiation is referred to as electromagnetic interference (EMI) shielding.
The typical methods to shield electronic devices consist of surrounding the electronic components with a conductive barrier which reflects and/or absorbs the radiation. However, the problems associated with electromagnetic interference continue to challenge manufacturers due to the proliferation of devices, increasing clock speeds, and the increased density of electronic packaging resulting from continual size reductions in portable devices. The problems have been further compounded by the increased use of plastic housings. Plastics tend to cost and weigh less than alternative metal structures, are resistant to corrosion and provide much greater design flexibility. However, plastics are natural insulators and as such are generally transparent to electromagnetic radiation in the applicable frequency range. Electronic equipment manufacturers have therefore been forced to find ways to shield the plastic enclosures to protect the components inside and comply with applicable regulations.
When evaluated according to standard testing protocol, the shielding imparted by a particular structure is reported in decibels. The shielding effect is measured by the following equation:
Shielding effect (dB)=20 log (E.sub.1/E.sub.2)
Where:
-E.sub.1 is the receiving level when no shielding material is placed between the transmitting and receiving antennas, and
-E.sub.2 is the receiving level when a shielding material is placed between transmitting and receiving antennas.
20 dB is generally considered a minimum range for meaningful shielding. 30-60 dB can be sufficient to solve moderate problems. 60-90 dB represents excellent shielding solving moderate to severe problems.
Many different electromagnetic shielding methods exist. The simplest in concept is to select a metal housing or cabinet for the shield. The use of metal shields includes stamped shells or cabinets, zinc die castings, and sheet metal liners. Stamped shells can be a cost effective way to shield sensitive components inside a device. However, such shells do not necessarily protect the rest of the device from external sources of radiation and design flexibility is limited. Zinc die castings provide effective shielding, especially in high temperature environments. However, zinc die castings are costly, heavy, and design flexibility is limited. Sheet metal liners can be combined with the appeal of a plastic exterior, but sheet metal liners tend to be expensive and have to be attached to the plastic housing which complicates and lengthens the assembly process.
A number of methods have been developed to provide shielding to plastic components. Perhaps the simplest is the use of conductive paints, usually comprising copper or nickel powder or flake in a polymeric binder. These paints can be applied with simple, conventional spray painting equipment. For parts with simple designs, painting is a low cost shielding option due to low capital costs. The paints can usually be applied to most plastics although some require an adhesion promoting treatment. One major disadvantage of conductive painting is that it is a line of sight process making it difficult to successfully shield recessed or ribbed areas. Thin coatings also will not provide good shielding effectiveness so multiple coats may be required. This may be a particular problem at sharp comers where the well-known xe2x80x9cpull-awayxe2x80x9d characteristic of paints may lead to slot antenna effects. Finally, the multiple coats which may be required result in significant material overspray waste, environmental difficulties, and increased costs. It is also difficult to achieve consistent paint thickness over the surface of an article which can result in variations in shielding performance. Finally, conductive paints often suffer from problems with durability over time. Due to these drawbacks the use of conductive paint coatings has grown very slowly over the past several years.
Another method for imparting EMI shielding capability to plastic articles is vacuum metallizing. In the vacuum metallizing process, parts are masked if necessary and then placed on rotating fixtures inside a vacuum chamber. Inside the chamber a metal, usually aluminum, is heated and vaporized. The metal will then condense on the surface of the plastic. Vacuum metallizing is generally a batch process that is best suited for small to moderate sized parts with limited geometrical complexity. For example, U.S. Pat. No. 5,811,050 to Gabower teaches the use of vacuum metallizing to shield thermoformed plastic parts. Thermoforming is a process limited to parts of relatively simple geometry. Gabower reported shielding effectiveness values of up to 60 dB for vacuum metallized, thermoformed parts. Thus, for these types of parts, vacuum metallizing provides reasonable shielding effectiveness at a relatively moderate cost. A major disadvantage of vacuum metallizing is the need for special equipment which requires a significant capital investment as well as a high operator skill level. In addition, vacuum deposition of the relatively thick films required for EMI shielding can be complex and process sensitive, as discussed in the Gabower U.S. Pat. No. 5,811,050. Like spray painting, vacuum metallizing is generally a line of sight process which makes it difficult to successfully shield recessed or ribbed areas. Finally, a base coat or ionization treatment is often required between the plastic and aluminum to minimize surface defects, and a protective coating may be required to protect the vacuum deposited metal.
An alternative method for imparting EMI shielding to plastics is electroless plating. Electroless plating involves chemically coating a nonconductive surface such as a plastic with a continuous metallic film. Unlike conventional electroplating, electroless plating does not require the use of electricity to deposit the metal. Instead, a series of chemical steps involving etchants and catalysts prepare the non-conductive plastic substrate to accept a metal layer deposited by chemical reduction of metal from solution. The process usually involves depositing a thin layer of highly conductive copper followed by a nickel topcoat which protects the copper sublayer from oxidation and corrosion. The thickness of the nickel topcoat can be adjusted depending on the abrasion and corrosion requirements of the final product. Because electroless plating is an immersion process, uniform coatings can be applied to almost any configuration regardless of size or complexity without a high reliance on operator skill. Electroless plating also provides a highly conductive pure metal surface which results in relatively good shielding effectiveness. In addition, electrolessly plated parts can be subsequently electroplated, although electroplating is generally not used unless a part also has certain decorative or functional requirements. On average, the cost of electroless plating will be higher than vacuum metallizing and conductive paints. The many steps employing harsh and expensive chemicals make the process intrinsically costly and environmentally difficult. The process comprises many steps and is very sensitive to processing variables used to fabricate the plastic substrate, limiting applications to carefully molded parts and designs. It may be difficult to properly mold conventional plateable plastics using the rapid injection rates often required for the thin walls of electronic components. The rapid injection rates can cause poor surface distribution of etchable species, resulting in poor surface roughening and subsequent inferior bonding of the chemically deposited metal. The rates at which metals can be chemically deposited are relatively slow, leading to increased process times to deposit the thicknesses required for adequate shielding and abrasion resistance. Finally, selective metallizing can be difficult, especially on complex parts, since the electroless plating may tend to coat any exposed surface unless the overall process is carefully controlled. The conventional technology for metal plating on plastic (etching, chemical reduction, optional electroplating) has been extensively documented and discussed in the public and commercial literature. See, for example, Saubestre, Transactions of the Institute of Metal Finishing, 1969, Vol. 47, or Arcilesi et al., Products Finishing, March 1984.
Many attempts have been made to simplify the process of plating on plastic substrates. Some involve special chemical techniques to produce an electrically conductive film on the surface followed by electroplating. Typical examples of the approach are taught by U.S. Pat. No. 3,523,875 to Minkiei, U.S. Pat. No. 3,682,786 to Brown et. Al., and U.S. Pat. No. 3,619,382 to Lupinski. The electrically conductive surface film produced was then electroplated.
Another approach proposed to simplify electroplating of plastic substrates is incorporation of electrically conductive fillers into the resin to produce an electrically conductive plastic which is then electroplated. In a discussion of polymers rendered electrically conductive by loading with electrically conductive fillers, it may be important to distinguish between xe2x80x9cmicroscopic resistivityxe2x80x9d and xe2x80x9cbulkxe2x80x9d or xe2x80x9cmacroscopic resistivityxe2x80x9d. xe2x80x9cMicroscopic resistivityxe2x80x9d refers to a characteristic of a polymer/filler mix considered at a relatively small linear dimension of for example 1 micrometer or less. xe2x80x9cBulkxe2x80x9d of xe2x80x9cmacroscopic resistivityxe2x80x9d refers to a characteristic determined over larger linear dimensions. To illustrate the difference between xe2x80x9cmicroscopicxe2x80x9d and xe2x80x9cbulk, macroscopicxe2x80x9d resistivities, one can consider a polymer loaded with conductive fibers at a fiber loading of 10 weight percent. Such a material might show a low xe2x80x9cbulk, macroscopicxe2x80x9d resistivity when the measurement is made over a relatively large distance. However, because of fiber separation (holes) such a composite might not exhibit consistent xe2x80x9cmicroscpicxe2x80x9d resistivity.
When considering producing an electrically conductive polymer intended to be electroplated, one should consider xe2x80x9cmicroscopic resistivityxe2x80x9d in order to achieve uniform, xe2x80x9chole-freexe2x80x9d deposit coverage. Thus, the fillers chosen will likely comprise those that are relatively small, but with loadings sufficient to supply the required conductive contacting. Such fillers include metal powders and flake, metal coated mica or spheres, conductive carbon black and the like. The metal containing fillers generally will have a relatively high cost, greatly increasing the volumetric cost of the final conductive material. Thus the use of metal containing fillers has been generally confined to preparation of paints and coatings and the like.
The least expensive (and least conductive) of the readily available conductive fillers for plastics are carbon blacks. Attempts have been made to produce electrically conductive polymers based on carbon black loading intended to be subsequently electroplated. Examples of this approach are the teachings of Adelman in, U.S. Pat. No. 4,038,042 and Luch in U.S. Pat. No. 3,865,699. Adelman taught incorporation of conductive carbon black into a polymeric matrix to achieve electrical conductivity required for electroplating. The substrate was pre-etched in chromic/sulfuric acid to achieve adhesion of the subsequently electrodeposited metal. Luch taught incorporation of small amounts of sulfur into polymer-based compounds filled with conductive carbon black. The sulfur was taught to have two advantages. First, it participated in formation of a chemical bond between the polymer-based substrate and an initial Group VIII based metal electrodeposit. Second, the sulfur increased lateral growth of the Group VIII based metal electrodeposit over the surface of the substrate.
Since the polymer-based compositions taught by Luch could be electroplated directly without any pretreatment, they could be accurately defined as directly electroplateable resins (DER). Directly electroplateable resins, (DER), are characterized in this specification by the following features.
(a) having a polymer matrix;
(b) presence of carbon black in amounts sufficient to have a xe2x80x9cmicroscopicxe2x80x9d electrical volume resistivity of the polymer/carbon black mix of less than 1000 ohm-cm., e.g. 100 ohm-cm., 10 ohm-cm., 1 ohm-cm.
(c) presence of sulfur (including any sulfur provided by sulfur donors) in amounts greater than about 0.1% by weight of the overall polymer-carbon black-sulfur composition; and
(d) presence of the polymer, carbon and sulfur in the directly electroplateable composition in cooperative amounts required to achieve direct, uniform, rapid and adherent coverage of said composition with an electrodeposited Group VIII metal or Group VIII metal-based alloy.
Polymers such as polyvinyls, polyolefins, polystryrenes, elastomers, polyamides, and polyesters were mentioned by Luch as suitable for a DER matrix, the choice generally being dictated by the physical properties required.
The minimum workable level of carbon black required to achieve xe2x80x9cmicroscopicxe2x80x9d electrical resistivities of less than 1000 ohm-cm. for the polymer/carbon black mix appears to be about 8 weight percent based on the combined weight of polymer plus carbon black. The xe2x80x9cmicroscopicxe2x80x9d material resistivity generally is not reduced below about 1 ohm-cm. by using conductive carbon black alone. This is several orders of magnitude larger than typical metal resistivities. The xe2x80x9cbulk, macroscopicxe2x80x9d resistivity of conductive carbon black filled polymers can be further reduced by augmenting the carbon black filler with additional highly conductive fillers such as metal containing fibers or flake. As will be taught below, this augmentation can be considered in designing the shielding composites of the present invention.
Due to multiple performance problems associated with their intended end use, none of the attempts to simplify the process of plating on plastic substrates identified above ever achieved any recognizable commercial success.
Spray painting, vacuum metallizing, electroless plating and electroplating are all secondary operations. Efforts have been made to eliminate these secondary operations in shielding applications by making the bulk plastic itself sufficiently conductive to impart required shielding performance. This technology generally involves mixing of the conductive filler into the polymer resin prior to fabrication of the material into its final shape. The conductive fillers typically consist of very high aspect ratio particles such as metal fibers to minimize adverse processing and property affects and high cost associated with required loadings when using fillers of lower aspect ratios (for example metal powders).
The most effective and widely employed fibers are stainless steel fibers and nickel coated graphite fibers. Properly molded conductive fiber loaded plastics offer good shielding performance but suffer from a number of major downfalls. The first is the need to maintain integrity and even distribution of the conductive fibers in the plastic matrix during compounding and fabrication. High shear processing can result in the breakage of the fibers and reduction in the material conductivity. It also may be difficult to maintain uniformity of fiber concentration in a final molded part. A further problem is the need to maintain good surface conductivity. Conductive fiber filled plastics can exhibit poor surface conductivity even though bulk conductivity is acceptable. This xe2x80x9cskinxe2x80x9d effect can result in slot antennas at mating surfaces and prevent additional processing, if desired or necessary, requiring such surface conductivity. The fiber loadings required can negatively impact the aesthetic quality of the surface finish. Finally, the cost of the fibrous filler is very high, often negating the savings achieved through the elimination of secondary processing.
Efforts have been made to overcome the reduced surface conductivity often associated with conductive fiber filled plastics. For example, U.S. Pat. No. 4,596,670 to Liu teaches compositions containing polymer binder, metal flake, conductive fibrous fillers and conductive carbon black. However, these heavily loaded compositions can be difficult to process and other material properties are negatively impacted. Further, these compositions do not eliminate the other problems associated with using fibers mentioned above.
Another widely used material form in the art of electromagnetic (EMI) shielding consists of electrically conductive meshes or webs. These structures generally comprise conductive fibers, normally metal containing, fabricated into a mesh or screen by knitting, braiding or non-woven fabric technology. Alternatively, metal foils with a myriad of punched holes can be considered.
Metal containing fiber mesh structures have been used for many years to provide EMI shielding for openings in enclosures to allow an air flow for cooling and ventilation.
Another direction of development to provide EMI shielding to plastic components is incorporation of the conductive fabrics, webs, or meshes into a final composite article. The general concept is to confine the expensive conductive filler to an essentially sheetlike structure and thereby eliminate the need to have conductivity throughout the bulk volume of the article. Thus, introduction of such a conductive web during fabrication of the article conceptually eliminates secondary operations while eliminating the difficulties and expense associated with imparting bulk conductivity throughout the entire volume of the article. One form of this technology involves production of a metal mesh or web by weaving or hole punching a thin foil. This technique is taught in U.S. Pat. No. 5,473,111 to Hattori et al. and U.S. Pat. No. 6,054,647 to Ridener. Hattori taught a shield member of planar plate-shaped wire netting made of copper wires (or the like) 30 to 60 micrometers in diameter or a copper foil having a plurality of small holes. A precut shield member was placed inside a mold cavity such that a portion of the member extends from the outer peripheries of the cavity upon closure of the mold. Unfortunately, injection of plastic resin into a mold cavity typically involves high pressures and shear which can cause destruction to fragile netting or foils. Hattori proposed to solve this by injecting a portion of the resin prior to mold closing and carefully monitoring the rate of mold closure to achieve a partial xe2x80x9ccompression moldingxe2x80x9d process. Clearly, this would be impractical except for the very simplest of geometries.
Ridener taught weaving a grid from nickel coated steel fibers and then annealing and calendering to bond the fibers together at the cross-over points of the grid. The bonding was taught to provide increased integrity and conductivity to the grid, allowing it to be used as an insert in an injection molding operation. Unfortunately, the combined operations of nickel plating, drawing, weaving, annealing and sintering involved in the grids taught considerably adds to manufacturing cost. In addition, the insert molding involves preforming of the insert and considerable restriction of the design aspects of the part.
Another form of the conductive web technology envisions formation of a non-woven fabric by combining electrically conductive fibers bound together with resinous fibers. For example, U.S. Pat. No. 4,939,027 to Daimon et al. teaches manufacture of an electroconductive thermoplastic resin sheet comprising a non-woven fabric composed mainly of electroconductive fibers and heat-meltable fibers laminated between first and second thermoplastic films. Stainless steel fibers were taught as one example of the conductive fibers used to produce the non-woven fabric. The Daimon patent reports EMI shielding as high as 60 dB (at 300 MHz) for the conductive sheets taught. U.S. Pat. No. 4,943,477 to Kanamura et al. taught production of a conductive sheet by combining chemically metal plated fibers with heat fusible fibers to form a non-woven fabric which was subsequently hot-press molded to effect a monolithic fused sheet. Kanamura reported electric shielding effectiveness as high as 70 dB (at 1000 MHz) for the sheets taught in the patent.
The conductive fibers taught by Daimon and Kanamura are expensive and the processing to produce the precursor conductive non-woven sheets is complicated. In addition, the reasonably high shielding values reported by Daimon and Kanamura were obtained on essentially flat sheets of the composites taught. However, a significant problem arises when one attempts to form more useful three dimensional articles from such sheets, either through stamping or thermoforming. The resin flow and expansion involved in these operations tends to separate and fracture the fibers, resulting in loss of fiber to fiber contact and conductivity necessary to achieve shielding. Thus, the shielding of the Daimon and Kanamura composites would be expected to be drastically reduced should those composites be formed into three dimensional articles. This problem was addressed by U.S. Pat. Nos. 5,028,490, 5,124,198, and 5,226,210 to Koskenmaki et al. and U.S. Pat. Nos. 5,869,412, 6,013,376 and 6,090,728 to Yenni, Jr. et al. One of the major goals of this group of patents is production of metal/polymer composite sheets which are thermoformable without significant deterioration of shielding quality. The metal is present in the form of fine, randomly oriented metal fibers forming a mat supported by a polymeric substrate or carrier sheet. The metal fibers are made of metals and alloys having melting points less than the temperatures required to soften the polymeric components sufficiently to carry out the thermoforming operation. In this case, fibers are molten and rupture is minimized during the dimensional expansion of the sheet during the thermoforming operation. The patents reported shielding effectiveness of a thermoformed article (metal fiber loading of 242 grams per square meter) ranging from 48-58 dB over the range 30-1000 MHz. A problem is that the low melting point metal fibers employed are relatively expensive and manufacture of the precursor sheets is complicated, thereby significantly increasing the cost of the final shaped article. In addition, significant amounts of trim waste of the expensive sheets can be associated with the thermoforming operation, further increasing cost.
Further general problems with the use of conductive mats or sheets as shields exist in that they normally end up embedded in the final article, resulting in a non-conductive surface. This can result in loss of electrical communication along the edges of mating parts and production of a xe2x80x9cslot antennaxe2x80x9d. Further, special operations are required if electrical communication must be achieved to other components of the device (for example the ground plane of a circuit board). Special connectors can be used, but at added cost. The Hattori U.S. Pat. No. 5,473,111 discussed above proposed to address this problem by extending the shield member outside the periphery of the resin portion of the part, but this is clearly impractical in most cases. Yenni et al. U.S. Pat. No. 6,090,728 taught exterior electrical connection by confining the insulating resinous layer covering the metal fiber mat to a thickness less than 0.15 mm. Using a specially designed and precisely controlled heat staking die, electrical connection to exterior conductive components was achieved. The unique design of the part and die along with the precise heat control required limits the application of this technique. In addition, the heat staking technique does not address the additional xe2x80x9cslot antennaxe2x80x9d problem.
There is thus a need to improve the xe2x80x9cconductive matxe2x80x9d technology to avoid the use of very expensive metal fibers currently employed and simplify the elaborate processing currently necessary for production of the precursor materials.
Further teachings regarding EMI shielding comprising conductive webs or meshes involves the design and manufacture of high performance shielding gaskets. The outer covers of electronic equipment are usually formed by combining more than one housing component. To provide the entire cover with EMI shielding properties, electrical continuity must be achieved among the multiple components. In addition, should the mating surfaces of housing components not contact adequately, a gap is formed in the joint between housing components. Leakage of electromagnetic radiation can occur through this gap.
Electrical continuity among multiple housing components can be achieved in a number of ways. As pointed out in U.S. Pat. No. 5,170,009 to Kadokura, methods such as lead wires, application of silver paste, spray coating of conductive paints and use of damping screws have been employed. All of these techniques have characteristic disadvantages. Kadokura in U.S. Pat. No. 5,170,009 taught simultaneously electrodepositing a conductive coating comprising fine conductive particles and an electroplateable resin base. The coating was sufficiently conductive and conformable to act as a xe2x80x9cconductive gasketxe2x80x9d between mating surfaces. The many steps, expensive materials and the fact that the entire component has to be coated with the specialized electrodeposited coating severely limit applications of the technique.
Shielding gaskets are known which comprise resilient materials such as elastomers filled with a conductive filler. For example, U.S. Pat. No. 4,662,967 to Bogan et al. describes a shielding gasket comprising conductive filler mixed with an elastomer and bonding agent. The gasket is bonded to a surface by curing the gasket material in contact with the surface.
As electronic equipment has evolved, higher frequencies are encountered, and the electrical conductivity imparted by conductive fillers becomes only marginally effective for adequate attenuation of the high frequency spectrum of the radiation. Thus, where such higher frequencies are encountered, it becomes necessary to utilize conductive mesh materials supported on a resilient core material such as an elastomer or foam to provide adequate shielding levels. Shielding structures comprising metal mesh supported by an internal resilient core are taught in U.S. Pat. Nos. RE33,256, 5,028,739, 5,386,345, and 5,142,101. A common feature of these higher performance gaskets is the requirement of providing some form of metal fiber mesh. These meshes utilize fibers made of relatively expensive materials such as stainless steel, Monel, tin-coated copper, or metal plated polymeric fibers. In addition, processing of the individual fibers into the mesh adds considerably to the overall cost of the gasket.
It is clear that the shielding techniques using metal-containing meshes, whether aimed at shielding ventilation openings, incorporation into final plastic components or for use in gasketing, could be greatly advanced by improved processing techniques for production of the mesh structures and the use of a minimum of expensive materials.
In order to eliminate ambiguity in terminology of the present specification and claims, the following definitions are supplied.
xe2x80x9cMetal-basedxe2x80x9d refers to a material having metallic properties comprising one or more elements, at least one of which is an elemental metal.
xe2x80x9cMetal-based alloyxe2x80x9d refers to a substance having metallic properties and being composed of two or more elements of which at least one is an elemental metal.
xe2x80x9cPolymer-basedxe2x80x9d refers to a substance composed, by volume, of 50 percent or more polymer.
xe2x80x9cGroup VIII-basedxe2x80x9d refers to a metal (including alloys) containing, by weight, 50% to 100/o metal from Group VIII of the Periodic Table of Elements.
An object of the invention is to produce unique and novel structures for achieving exceptional shielding of electromagnetic radiation.
A further object of the invention is to teach improved and simplified processes for producing electromagnetic interference (EMI) shields.
A further object of the invention is to provide low cost materials and composites capable of being fabricated into EMI shields using multiple different processes.
These and other objects of the invention will become clear in light of the accompanying figures and description of the preferred embodiments.
The instant invention contemplates employing the uniquely advantageous characteristics of directly electroplateable resins to provide exceptional electromagnetic interference (EMI) shielding articles.